In the field of paper manufacturing, a large amount of a filler (for example, talc, clay) is normally employed in order to improve printability and whiteness and to impart opacity. In recent years, attempts have been made to substitute these conventional fillers with calcium carbonate which exists abundantly in Japan and is commercially available at a low price.
Suitable, commonly employed paper sizing agents include so-called anion type sizing agents such as rosin sizing agents which are normally fixed in pulp with the use of aluminum sulfate, namely, the acidic sizing method. When employed in this method as a filler, however, calcium carbonate is at least partially decomposed due to the acidic paper manufacturing system.
In order to solve this problem, therefore, various neutral sizing agents, capable of performing sizing within a neutral or alkaline region without using aluminum sulfate, have been proposed. For example, JP-B-39-2305 (corresponding to U.S. Pat. No. 3,102,064) discloses a neutral sizing agent represented by the following general formula (a) (the term "JP-B" as used herein means an "examined Japanese patent publication"): ##STR2## wherein R represents a group selected from a dimethyl group and a trimethyl group; and R' represents a group selected from an alkyl group, an alkenyl group, an aralkyl group and an aralkenyl group having 5 or more carbon atoms. JP-B-53-28526 (corresponding to U.S. Pat. No. 3,821,069) discloses a sizing agent comprising a product obtained by a reaction between an internal olefin represented by the following general formula (b): EQU R.sub.x --CH.sub.2 --CH.dbd.CH--CH.sub.2 --R.sub.y (b)
wherein R.sub.x and R.sub.7 each represent an alkyl group having from 4 to 10 carbon atoms; and maleic anhydride.
Further, JP-A-57-154495 discloses a sizing agent comprising mixed alkenylsuccinic anhydrides which are obtained by adding maleic anhydride to a C.sub.8 -C.sub.18 straight-chain internal olefin mixture wherein double bonds are almost uniformly distributed at all positions except the .alpha.-position (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Furthermore, JP-A-59-179898 discloses a sizing agent containing a product obtained by an addition reaction between a C.sub.14 -C.sub.36 straight-chain olefin mixture, wherein the content of olefins having a double bond at the 2-, 3- or 4-position is each from 10 to 65 mol% and the total content of these olefins is at least 70 mol%, with maleic anhydride or a hydrogenated product of the reaction product.
Furthermore, JP-A-60-99099 involves discussion on emulsifiers.
The above-mentioned conventional sizing agents are emulsified, for example, in a homomixer or a homogenizer with the use of a water soluble polymer compound such as cationized starch or a surfactant such as polyoxyalkylene aryl ether and then added, in the form of an aqueous emulsion, to a pulp slurry. However each of these sizing agents is insufficient in emulsifiability and stability after the completion of the emulsification, which results in certain problems including an unsatisfactory sizing effect, or contamination in the paper manufacturing system.
It is considered that the above-noted disadvantages may arise because a conventional alkenylsuccinic anhydride type sizing agent has a relatively wide size distribution of emulsified particles. In other words, fine particles of a particle size of 0.5 .mu.m or less would undergo hydration within a short period of time following the addition to the pulp slurry and, as a result, the emulsion is broken. Therefore these fine particles never contribute to the achievement of the sizing effect but instead cause contamination in the paper production system. On the other hand, it is considered that large particles of a particle size exceeding 2 .mu.m would scarcely contribute to the sizing effect, since the pulp surface area to be coated with these particles is limited.
In order to prevent the contamination of the paper manufacturing system and to achieve an excellent sizing effect, it is, therefore, desirable to use a sizing agent emulsion of a narrow size distribution of emulsified particles (i.e., around 1 .mu.m). However, the prior art, i.e., by improving, for example, the structure of starting olefins, emulsifiers or emulsification procedures, does not teach or suggest an effective solution to this problem.